Beside their technological applications, colloids have provided us with tunable model systems for exploring thermodynamic and kinetic phenomena of fundamental value. The hard-sphere crystallization, the glass transition, the range-controlled thermodynamic existence of the liquid state are beautiful examples of exploiting the "large atom" nature of colloids. In the last ten years, a significant effort has been put in the direction of providing valence to colloids, to move from colloidal atoms to colloidal molecules. Many experimental, theoretical and numerical efforts have been devoted to the synthesis and to the investigation of these anisotropic colloids.

In the talk I will focus on exploiting the possibility of controlling the interaction between colloidal particles to tackle fundamental issues in the molecular world. In particular I will discuss how some recent experimental test of the role of the valence in controlling the gas-liquid transition, the dynamics of formation of an equilibrium gel, and the thermodynamic stability of the gel phase respect to crystallization. I will show that for patchy colloids with limited valence, conditions can be found for which the liquid phase is stable even in the zero-temperature limit. Limited valence colloids appear to be good candidates for providing insights in the dynamic arresting behavior of strong network-forming liquids, like silica and silicon.

Finally, I will present some very recent work where we elucidate, using colloidal models, the possibility of a liquid-liquid transition in water.